Enhancement of ionospheric heating effect by chemical release

The ionosphere can be artificially modified by employing ground-based high-power high-frequency electromagnetic waves to irradiate the ionosphere. This modification is achieved through the nonlinear interaction between the electromagnetic waves and the ionospheric plasma, leading to changes in the physical properties and structure of the ionosphere. The degree of artificial modification of the ionosphere is closely related to the heating energy density of high-frequency pump waves. Due to the high density of neutral constituents in the lower ionosphere and the high frequency of electron-neutral collisions, the energy of heating pump waves will be absorbed and attenuated during the penetration of the low ionosphere, seriously affecting the heating effect. This paper proposes a method to reduce the absorption of ionospheric heating pump waves by releasing electron attachment chemicals into low ionosphere to form a large-scale electron density hole. A model for mitigating pump waves absorption based on SF6 release is established, and the absorption at different frequencies is quantitatively calculated. The propagation characteristics of high-frequency signals in ionospheric holes are studied using a three-dimensional ray tracing method, and the results demonstrate that the chemical release method not only reduces the absorption attenuation of heating pump waves but also forms spherical electron density holes, which exhibit a focusing effect on the heating beam and enhance the heating effect. The results are of great significance for understanding the nonlinear interaction between electromagnetic wave and ionospheric plasma and improving the ionospheric heating efficiency.

This study addresses the demand for mitigating the absorption effects of high-frequency radio wave heating in the lower ionosphere.A large number of studies and experiments have been carried out on ionospheric hole and electron density irregularities caused by the chemical release [18][19][20][21] .However, there are no relevant reports on the use of electrons attachment by chemical release to achieve low ionospheric absorption reduction of heat pump beam.Hence, we propose a method to mitigate the absorption of ionospheric heating pump waves by releasing electron attachment chemicals at low ionospheric altitudes.A model for mitigating ionospheric heating absorption based on SF 6 release is established, and the absorption of pump waves at different frequencies is quantitatively calculated.The propagation characteristics of high-frequency signals in ionospheric holes are studied using a three-dimensional ray tracing method, and the results demonstrate that the chemical release method not only reduces the absorption attenuation of heating pump waves but also forms spherical electron density holes, which exhibit a focusing effect on the heating beam and enhance the heating effect.The research of this study provides important theoretical support for studying the nonlinear interaction processes between electromagnetic waves and ionospheric plasma and improving the efficiency of ionospheric heating.

The low ionospheric chemical release theory
Ionospheric chemical release refers to the injection of chemicals into ionospheric altitudes using sounding rockets, satellites, and spacecraft, among other launch vehicles.This artificial manipulation modifies the composition and structure of the ionospheric plasma, resulting in significant short-term changes in the ionosphere [22][23][24][25] .The release of ionospheric chemicals holds significant importance for the in-depth investigations into the dynamics and coupling mechanisms of the ionospheric system, the study of ionospheric instability excitation mechanisms and processes, and the construction of a new instability theory system.

Chemicals diffusion
In the initial stage of chemical release, under the influence of pressure, the release pushes the surrounding plasma away like a snowplow.This process occurs at supersonic speeds and lasts only a few seconds.Subsequently, the pressure difference rapidly decreases until it becomes comparable to the background pressure.At this point, the released material and the surrounding plasma mix thoroughly and begin to diffuse into space.This diffusion process takes a long time and in which the ionic chemical reaction mainly occurs.
The diffusion equation is expressed as: where D is the diffusion coefficient, n i (x, y, z, t) is the number density of releases and satisfies n i (x, y, z, 0) = N 0 δ(x, y, z) , N 0 is the total number of molecules released.
Considering the release at the beginning of the release as a point source.Under the assumption of a stratified background ionosphere and thermosphere, the diffusion process of the released material can be approximated by the following equation 23 : where n i (r, z, t) is the density of released material as a function of time t and space (r and z are the radial distance from the point source and the altitude of the ionosphere, respectively), z 0 is the altitude of the point of release, N 0 is the total number of molecules released, and D 0 is the release point diffusion coefficient.H α = kT/m a g is the atmospheric scale altitude and H i = kT/m i g is the released gas scale altitude, where k is the Boltzmann constant, T is the neutral gas temperature, m a and m i are the average molecular weight of the atmosphere and the molecular weight of the released gas, respectively, g is the gravitational acceleration, and αt is the loss term due to chemical reactions.

Chemical reaction of releases with the ionosphere
The main chemical reaction processes of SF 6 in the ionospheric plasma are shown in Table 1.Among the four equations in Table 1, the reaction efficiency of Eqs.(3) and ( 4) is very low due to the low O + density at 100 km and 120 km altitude, and they belong to the secondary reactions.In Eqs.(1) and (2), Eq. ( 2) is more important than Eq. 1, because the reaction coefficient k 2 of Eq. ( 2) is much larger than k 1 .

Plasma diffusion
The change in electron density in the release region of ionospheric chemicals disrupts the existing density distribution structure and dynamic equilibrium of charged particles.According to plasma diffusion theory, the transport equation for plasma can be derived as follows 27 : (1) (2) where n p is the density of ions or electrons; P p and L p represent the production and recombination rates of charged particles, respectively.
, where L 0 is the loss rates of O + reacting with other particles and photochemical reactions, the released neutral gas consists of M species of neutral molecules, and n i represents the chemical reaction rate between species i and O + ; T p = (T e + T i )/2 is the plasma temperature; H p is the plasma scale altitude , H p = 2T p k/(m p g) ; I is the magnetic inclination angle, γ is the magnetic declination Angle; D is the effective bipolar diffusion coefficient, D = (1 + T e /T i )D i , where D i is the ion diffusion coefficient; v D represents the applied drift velocity (wind speed).

Simulation of ionospheric depletion region generation and evolution
In order to study the method of eliminating the absorption of high frequency radio waves in the lower ionosphere by releasing electron-attached chemicals, the background ionosphere generated by IRI2020 model at 12:00 local time on April 15, 2022 in Hainan region was numerically simulated.The ionospheric disturbance effects resulting from the release of 30 kg of SF 6 at altitudes of 100 km and 120 km were simulated.As shown in Fig. 1 and Fig. 2.
From the simulation results, it can be observed that the maximum ionospheric hole diameter formed by the release of 30 kg SF 6 at an altitude of 100 km is approximately 25 km.The maximum ionospheric hole diameter formed by releasing 30 kg SF 6 at an altitude of 120 km is about 30 km, and the electron density is close to complete depletion.It should be noted that when releasing chemicals at low ionospheric altitudes, the smaller diffusion coefficient of the released material results in smaller ionospheric hole scales, requiring higher control of the release point in space experiments.

Calculation of low ionospheric absorption attenuation for typical time periods
According to the absorption attenuation theory, the total absorption attenuation L of the lower ionosphere can be expressed as 28 : where n e is the electron density, ε 0 is the vacuum permittivity, c is the speed of light, µ is the magnetic perme- ability, m is the electron mass, e is the elementary charge, v is the collision frequency, w is the incident wave frequency, and S is the propagation distance.
The collision frequency v can be expressed as 29 : (3)  26 .

Index
Reaction equation Reaction rate (cm 3 /s)  where n is the neutral component density (cm −3 ) and T is the temperature.
According to the atmosnrlmsise00 model, the altitude profiles of neutral component density and temperature are calculated.The following seven neutral molecules are considered in the calculation process: He, O, N 2 , O 2 , Ar, H and N. Based on this, the profile of electron-neutral molecule collision frequency is computed, as depicted in Fig. 3.
Based on the theory of absorption attenuation in the low ionosphere and the above analyses, the altitude distribution of absorption coefficients at 5 MHz and 7 MHz is calculated as shown in Fig. 4. By integrating the absorption coefficients over altitude, the total absorption attenuation is 4.77 dB for 5 MHz and 2.43 dB for 7 MHz in the altitude range of 40 − 240 km.The maximum absorption altitude varies under different ionospheric backgrounds (different locations, seasons, solar activity) and heating frequencies.To enhance the heating effect using electron-captured material release methods, the release altitude of the material needs to be optimized based on the ionospheric background conditions and heating frequency.

Evaluation of absorption elimination effect in the low ionosphere
At an altitude of 100 km, after a time period of 600 s following the release, the total absorption attenuation is calculated to be 1.04 dB for the 5 MHz incident wave and 0.53 dB for the 7 MHz incident wave in the altitude range of 40-240 km.The distribution of absorption attenuation coefficients is illustrated in Fig. 5.
At an altitude of 120 km, 600 s after release, using the same calculation method for absorption coefficients as mentioned above, the total absorption of 5 MHz incident wave is calculated to be 4.16 dB, and the total absorption  of 7 MHz incident wave is calculated to be 2.12 dB in the range of 40-240 km altitude.The distribution of absorption attenuation coefficients is shown in Fig. 6.Heating antenna 3 dB beamwidth: 19.5/24@5 MHz, 17/21.8@7MHz.Based on this, the antenna beam diameter at an altitude of 100 km is approximately 35 km@5 MHz and 33 km@7 MHz.At an altitude of 120 km, the antenna beam diameter is approximately 42 km@5 MHz and 40 km@7 MHz.The ionospheric holes created by the release at 100 km have a diameter of approximately 25 km, while at 120 km, the ionospheric holes have a diameter of approximately 30 km.The scale of the ionospheric holes can cover about 70-80% of the heating antenna beam.

Radio wave propagation in the modulated ionosphere
In order to further investigate the impact of the ionospheric depletion structure generated by SF 6 release on the information transmission link, shortwave ray tracing was conducted to simulate the changes in the propagation path and direction of the shortwave signals at different frequencies after traversing the depletion region.Ray tracing is an effective method for studying the ionospheric wave propagation, especially for radio wave signals above the high frequency band.It can accurately describe the propagation paths of electromagnetic waves [30][31][32] .In this section, a three-dimensional numerical ray tracing method is used to study the propagation characteristics of shortwave signals in the artificial depletion region.
To further investigate the propagation characteristics of the heating pump wave beam in controlling the ionosphere, based on the half-power beamwidth of the heating pump wave beam, the short-wave ray tracing   www.nature.com/scientificreports/results of the 4-9 MHz ionospheric heated pump wave at an altitude of 100 km and 120 km with the release of 30 kg of SF 6 are given in Figs. 7, 8, respectively, at 1000 s after the release.From Figs. 7, 8, it can be observed that by releasing electron attachment chemicals such as SF 6 at low ionospheric altitudes, a large-scale electron-depleted region is formed, which can cover more than 80% of the heating beam area.This not only reduces the absorption attenuation of the heating pump wave but also, due to the formation of a spherical electron-density hole, the refractive index inside the hole is lower than that of the surrounding plasma.As a result, the heating pump wave beam becomes concentrated in a smaller area, leading to increased heating energy density.This is beneficial for generating a focusing heating effect and enhancing the overall heating efficiency.

Conclusion and discussion
Enhancing the efficiency of ionospheric heating is one of the important research topics in ionospheric heating technology.It is expected that the absorption effect of high-frequency pump wave energy in the low ionosphere can be greatly reduced by releasing neutral gas into the low ionosphere to form ionospheric electron density holes.This paper proposes, for the first time, a method to reduce the absorption of ionospheric heating pump waves by releasing electron attachment chemicals at low ionospheric altitude s to form a large-scale electron density hole.A model for mitigating the absorption of ionospheric heating based on SF 6 release is established, and the absorption mitigation effects of different frequency pump waves are quantitatively calculated.The propagation characteristics of short-wave signals in ionospheric holes are studied using a three-dimensional ray tracing method, and the results show that the method of releasing electron attachment chemicals not only reduces the absorption attenuation of the heating pump waves but also creates spherical electron density holes that contribute to the generation of a focusing heating effect, thereby enhancing the overall heating efficiency.By combining ionospheric heating and ionospheric chemical release, this study explores approaches and methods to improve the efficiency of ionospheric heating.The results are of great significance for studying the nonlinear interaction between electromagnetic wave and ionospheric plasma and improving the heating efficiency of the ionosphere.
The following conclusions are obtained from theoretical modelling and simulation calculations: (1).Releasing SF 6 gas at an appropriate altitude can significantly reduce the absorption attenuation of highfrequency electromagnetic waves in the low ionosphere by creating ionospheric electron density holes.
(2).The maximum absorption altitude varies under different ionospheric backgrounds (different locations, seasons, solar activity) and heating frequencies, and the release altitude of the chemical needs to be optimized based on the ionospheric background conditions and heating frequency.(3).Due to the small diffusion coefficient of the released chemical, the ionospheric hole scale is small, so it is necessary to control the release location precisely in the space experiment of the low ionospheric chemical release.(4).By releasing electron attachment chemicals such as SF6 at low ionospheric altitudes, a large-scale electrondensity depletion region is formed.This method reduces the absorption attenuation of the heating pump waves.Additionally, the formation of spherical electron-density hole results in a lower refractive index compared to the surrounding plasma.As a result, the heating pump wave beam becomes concentrated in a smaller area, which can increase the heating energy density.This is beneficial for generating a focusing heating effect and enhancing the overall heating efficiency.

Figure 1 .
Figure 1.Evolution of the effect of releasing 30 kg SF 6 at 100 km altitude.

Figure 2 .
Figure 2. Evolution of the effect of releasing 30 kg SF 6 at 120 km altitude.

Figure 7 .
Figure 7. Propagation process of electromagnetic signals at different frequencies after releasing 30 kg SF 6 at an altitude of 100 km for 1000 s.

Figure 8 .
Figure 8. Propagation process of electromagnetic signals at different frequencies after releasing 30 kg SF 6 at an altitude of 120 km for 1000 s.

Table 1 .
Reactions stimulated by SF 6 release in ionosphere